#include<stdio.h>
typedef struct note
{
    int x;
    int s;
}note;
int main()
{
    note que[2501];
    int e[51][51]={0},book[51]={0};
    int head,tail;
    int i,j,n,m,a,b,cur,start,end,flag=0;
    scanf("%d%d%d%d",&n,&m,&start,&end);
    for(i=1;i<=n;i++)
        for(j=1;j<=n;j++)
            if(i==j)    e[i][j]=0;
            else        e[i][j]=99999999;
    for(i=1;i<=m;i++)
    {
        scanf("%d%d",&a,&b);
        e[a][b]=1;
        e[b][a]=1;
    }
    head=1,tail=1;
    que[tail].x=start;
    que[tail].s=0;
    tail++;
    book[start]=1;
    while(head<tail)
    {
        cur=que[head].x;
        for(i=1;i<=n;i++)
        {
            if(e[cur][i]!=99999999&&book[i]==0)
            {
                que[tail].x=i;
                que[tail].s=que[head].s+1;
                tail++;
                book[i]=1;
            }
            if(que[tail].x==end)
            {
                flag=1;
                break;
            }
        }
        if(flag==1)
            break;
        head++;
    }
    printf("%d",que[tail-1].s);
    return 0;
}